5 Tuning Java Virtual Machines (JVMs)

This chapter describes how to configure JVM tuning options for WebLogic Server. The Java virtual machine (JVM) is a virtual "execution engine" instance that executes the bytecodes in Java class files on a microprocessor. How you tune your JVM affects the performance of WebLogic Server and your applications.

Deployments using different JVM versions for the client and server are supported in WebLogic Server. See "Supported Configurations" in What's New in Oracle WebLogic Server for links to the latest supported mixed client/server JVMs.

UNIX threading models

Choices you make about Solaris threading models can have a large impact on the performance of your JVM on Solaris. You can choose from multiple threading models and different methods of synchronization within the model, but this varies from JVM to JVM.

Which JVM for Your System?

Although this section focuses on the Java SE 5.0 JVM for the Windows, UNIX, and Linux platforms, the JRockit JVM was developed expressly for server-side applications and optimized for Intel architectures to ensure reliability, scalability, manageability, and flexibility for Java applications. For more information about the benefits of using JRockit on Windows and Linux platforms, see "Introduction to JRockit JDK" at https://download.oracle.com/docs/cd/E13150_01/jrockit_jvm/jrockit/webdocs/index.html.

VM Heap Size and Garbage Collection

The Java heap is where the objects of a Java program live. It is a repository for live objects, dead objects, and free memory. When an object can no longer be reached from any pointer in the running program, it is considered "garbage" and ready for collection. A best practice is to tune the time spent doing garbage collection to within 5% of execution time.

The JVM heap size determines how often and how long the VM spends collecting garbage. An acceptable rate for garbage collection is application-specific and should be adjusted after analyzing the actual time and frequency of garbage collections. If you set a large heap size, full garbage collection is slower, but it occurs less frequently. If you set your heap size in accordance with your memory needs, full garbage collection is faster, but occurs more frequently.

The goal of tuning your heap size is to minimize the time that your JVM spends doing garbage collection while maximizing the number of clients that WebLogic Server can handle at a given time. To ensure maximum performance during benchmarking, you might set high heap size values to ensure that garbage collection does not occur during the entire run of the benchmark.

You might see the following Java error if you are running out of heap space:

Choosing a Garbage Collection Scheme

Depending on which JVM you are using, you can choose from several garbage collection schemes to manage your system memory. For example, some garbage collection schemes are more appropriate for a given type of application. Once you have an understanding of the workload of the application and the different garbage collection algorithms utilized by the JVM, you can optimize the configuration of the garbage collection.

Refer to the following links for in-depth discussions of garbage collection options for your JVM:

Using Verbose Garbage Collection to Determine Heap Size

The verbose garbage collection option (verbosegc) enables you to measure exactly how much time and resources are put into garbage collection. To determine the most effective heap size, turn on verbose garbage collection and redirect the output to a log file for diagnostic purposes.

The following steps outline this procedure:

Monitor the performance of WebLogic Server under maximum load while running your application.

Use the -verbosegc option to turn on verbose garbage collection output for your JVM and redirect both the standard error and standard output to a log file.

This places thread dump information in the proper context with WebLogic Server informational and error messages, and provides a more useful log for diagnostic purposes.

where the logfile.txt 2>&1 command redirects both the standard error and standard output to a log file.

On HPUX, use the following option to redirect stderrstdout to a single file:

-Xverbosegc:file=/tmp/gc$$.out

where $$ maps to the process ID (PID) of the Java process. Because the output includes timestamps for when garbage collection ran, you can infer how often garbage collection occurs.

Analyze the following data points:

How often is garbage collection taking place? In the weblogic.log file, compare the time stamps around the garbage collection.

How long is garbage collection taking? Full garbage collection should not take longer than 3 to 5 seconds.

What is your average memory footprint? In other words, what does the heap settle back down to after each full garbage collection? If the heap always settles to 85 percent free, you might set the heap size smaller.

Make sure that the heap size is not larger than the available free RAM on your system.

Use as large a heap size as possible without causing your system to "swap" pages to disk. The amount of free RAM on your system depends on your hardware configuration and the memory requirements of running processes on your machine. See your system administrator for help in determining the amount of free RAM on your system.

If you find that your system is spending too much time collecting garbage (your allocated virtual memory is more than your RAM can handle), lower your heap size.

Typically, you should use 80 percent of the available RAM (not taken by the operating system or other processes) for your JVM.

If you find that you have a large amount of available free RAM remaining, run more instances of WebLogic Server on your machine.

Remember, the goal of tuning your heap size is to minimize the time that your JVM spends doing garbage collection while maximizing the number of clients that WebLogic Server can handle at a given time.

Specifying Heap Size Values

System performance is greatly influenced by the size of the Java heap available to the JVM. This section describes the command line options you use to define the heap sizes values.You must specify Java heap size values each time you start an instance of WebLogic Server. This can be done either from the java command line or by modifying the default values in the sample startup scripts that are provided with the WebLogic distribution for starting WebLogic Server.

Tuning Tips for Heap Sizes

The following section provides general guidelines for tuning VM heap sizes:

The heap sizes should be set to values such that the maximum amount of memory used by the VM does not exceed the amount of available physical RAM. If this value is exceeded, the OS starts paging and performance degrades significantly. The VM always uses more memory than the heap size. The memory required for internal VM functionality, native libraries outside of the VM, and permanent generation memory (for the Sun VM only: memory required to store classes and methods) is allocated in addition to the heap size settings.

When using a generational garbage collection scheme, the nursery size should not exceed more than half the total Java heap size. Typically, 25% to 40% of the heap size is adequate.

In production environments, set the minimum heap size and the maximum heap size to the same value to prevent wasting VM resources used to constantly grow and shrink the heap. This also applies to the New generation heap sizes (Sun) or Nursery size (Jrockit).

JRockit JVM Heap Size Options

Although JRockit provides automatic heap resizing heuristics, they are not optimal for all applications. In most situations, best performance is achieved by tuning the VM for each application by adjusting the heaps size options shown in the following table.

Table 5-2 JRockit JVM Heap Size Options

TASK

Option

Description

Setting the Nursery

-Xns

Optimally, you should try to make the nursery as large as possible while still keeping the garbage collection pause times acceptably low. This is particularly important if your application is creating a lot of temporary objects.

The maximum size of a nursery cannot exceed 95% of the maximum heap size.

Setting a low maximum heap value compared to the amount of live data decrease performance by forcing frequent garbage collections.

Setting garbage collection

-Xgc: parallel

Performs adaptive optimizations as early as possible in the Java application run.

-XXaggressive:memory

To do this, the bottleneck detector will run with a higher frequency from the start and then gradually lower its frequency. This options also tells JRockit to use the available memory aggressively.

For example, when you start a WebLogic Server instance from a java command line, you could specify the JRockit VM heap size values as follows:

$ java -Xns10m -Xms512m -Xmx512m

The default size for these values is measured in bytes. Append the letter 'k' or 'K' to the value to indicate kilobytes, 'm' or 'M' to indicate megabytes, and 'g' or 'G' to indicate gigabytes. The example above allocates 10 megabytes of memory to the Nursery heap sizes and 512 megabytes of memory to the minimum and maximum heap sizes for the WebLogic Server instance running in the JVM.

Java HotSpot VM Heap Size Options

You achieve best performance by individually tuning each application. However, configuring the Java HotSpot VM heap size options listed in the following table when starting WebLogic Server increases performance for most applications.

These options may differ depending on your architecture and operating system. See your vendor's documentation for platform-specific JVM tuning options.

Table 5-3 Java Heap Size Options

Task

Option

Comments

Setting the New generation heap size

-XX:NewSize

As a general rule, set -XX:NewSize to be one-fourth the size of the heap size. Increase the value of this option for larger numbers of short-lived objects.

Be sure to increase the New generation as you increase the number of processors. Memory allocation can be parallel, but garbage collection is not parallel.

Setting the maximum New generation heap size

-XX:MaxNewSize

Set the maximum size of the New Generation heap size.

Setting New heap size ratios

-XX:SurvivorRatio

The New generation area is divided into three sub-areas: Eden, and two survivor spaces that are equal in size.

Configure the ratio of the Eden/survivor space size. Try setting this value to 8, and then monitor your garbage collection.

Setting initial heap size

-Xms

As a general rule, set intial heap size (-Xms) equal to the maximum heap size (-Xmx) to minimize garbage collections.

The default size for these values is measured in bytes. Append the letter 'k' or 'K' to the value to indicate kilobytes, 'm' or 'M' to indicate megabytes, and 'g' or 'G' to indicate gigabytes. The example above allocates 128 megabytes of memory to the New generation and maximum New generation heap sizes, and 512 megabytes of memory to the minimum and maximum heap sizes for the WebLogic Server instance running in the JVM.

Other Java HotSpot VM Options

Oracle provides other standard and non-standard command-line options to improve the performance of your VM. How you use these options depends on how your application is coded.

Automatically Logging Low Memory Conditions

WebLogic Server enables you to automatically log low memory conditions observed by the server. WebLogic Server detects low memory by sampling the available free memory a set number of times during a time interval. At the end of each interval, an average of the free memory is recorded and compared to the average obtained at the next interval. If the average drops by a user-configured amount after any sample interval, the server logs a low memory warning message in the log file and sets the server health state to "warning." See "Log low memory conditions" in Oracle WebLogic Server Administration Console Help.

Manually Requesting Garbage Collection

You may find it necessary to manually request full garbage collection from the Administration Console. When you do, remember that garbage collection is costly as the JVM often examines every living object in the heap. See "Manually request garbage collection" in Oracle WebLogic Server Administration Console Help.

Requesting Thread Stacks

Enable Spinning for IA32 Platforms

If you are running a high-stress application with heavily contended locks on a multiprocessor system, you can attempt to improve performance by using spinning. This option enables the ability to spin the lock for a short time before going to sleep.

Sun JDK

Sun has changed the default lock spinning behavior in JDK 5.0 on the Windows IA32 platform. For the JDK 5.0 release, lock spinning is disabled by default. For this release, Oracle has explicitly enabled spinning in the environment scripts used to start WebLogic Server. To enable spinning, use the following VM option:

-XX:+UseSpinning

JRockit

The JRockit VM automatically adjusts the spinning for different locks, eliminating the need set this parameter.

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